The present invention relates to communication systems and in particular relates to the time alignment of uplink CDMA signals.
Communication systems employing Code Division Multiple Access (CDMA), signals between a base station and a subscriber are transmitted over a frequency band of the particular communication channel with a particular subscriber spreading code. That is to say the signals in the communication channel for a particular frequency band of the communication channel are separated by these particular spreading codes. These particular subscriber spreading codes are preferably orthogonal with respect to each other such that a cross correlation between time aligned spreading codes is 0.
The orthogonality factor (OF) is a measure of how badly a multi-path channel degrades with the orthogonality of signals. An OF of 0 means that the signals remain completely orthogonal. An OF of 1 means that orthogonality is completely lost and communication performance would be unchanged if random sequences were transmitted rather than orthogonal sequences. A consequence of OF is that intra-cell multiple access interference (MAI) in a system employing orthogonal transmitted sequences will be reduced relative to that of a reference system employing random sequences.
In CDMA communications, individual transmissions are maintained orthogonal with respect to other transmissions on the same frequency by coding each transmission with a direct sequence pseudo-random (PN) code produced by a chip code generator which is supplied to a spread spectrum modulator along with the intermediate frequency (IF) from an oscillator. CDMA allows multiple simultaneous signals which completely overlap in time and frequency. Despite this overlap, the number of spreading codes allows each signal to be detected separately, with limited interference from the other signals. The level of interference is further reduced if orthogonal codes are used, and the channel OF is low. The channel OF can be minimised through time alignment of the strongest path.
The use of orthogonal sequences offers benefits in cases where different users symbols are received with time alignment. For flat channels, time aligning signals at the receiver results in perfect orthogonality and hence no intra-cell interference. In the more realistic case, where channels exhibit dispersive multi-path characteristics (i.e. the channels are not flat), each separate multi-path component will interfere with each other component which has a different delay. In this case it is only possible, in general, to align a single component from each signal, and orthogonality will be partially lost due to the interference between non-aligned terms. The OF achieved in any scenarios depend both upon the channels involved and the relative timing between users signals.
Referring now to FIG. 1 there is shown a simple situation where only a single multi-path component from each user can be simultaneously aligned (and each component is separated in time by at least one chip duration). In this situation, alignment of the strongest component of each profile will result in the lowest value achievable with those channels. The spacing of the multi-paths is such that only a single component from each channel can be aligned simultaneously.
In the three cases shown in FIGS. 1a-c, the different time alignments for the same two channel power delay profiles vary from no orthogonality in case a i.e. OF=1.0 to a case where the OF=0.85, as shown in case b. In case c the strongest component of each channel is aligned thus removing the largest cross interference term and reducing the OF still further to OF=0.55. In this example, it is only possible to align a single multi-path component between each pair of users and the optimum timing alignment between the users is simple to determine; the strongest multi-path component of each user""s signal should arrive simultaneously. In practice it is unlikely that more than a single multi-path component from each user""s channel can be aligned simultaneously. In addition, when fractional-chip delays between components are present, performance is also dependent upon the set shape of the chip wave form.
Referring now to ITU-A and ITU-B, (International Telecommunications Union) channel models, which were developed for mobile systems, each path independently fades according to a Rayleigh distribution. For an IS95 system, for example, the chip period is 813.8 ns, and for a wide band system with three times the bandwidth it is 271.3 ns. The channel power delay profiles are provided in table 1 and it can be seen that at both chip rates, both channels have components which are not separately resolvable.
FIGS. 2 and 3 demonstrate the respective A and B channel models (for the purposes of particular example, the individual path powers are held equal to their mean values, but the phases are allowed to vary randomly). Each trace is for a different set of phases of the multi-path components. The peaks in the band limited power delay profile do not directly correspond in time with those of any of the multi-path components for the non-band limited channel. For the ITU-A channel, most of the channel power is contained in the first tap and the remaining significant taps have very short delays. It can be seen that even if the power of the individual multi-path is constant, relative phase changes between them can cause the resultant power to vary significantly, even when one of the components is much stronger than the other. For the ITU-B channel, the first two paths are of comparable strength and are closely spaced relative to the chip duration. If they have opposite phases then they will combine destructively resulting in very low output power for time delays in that region. The second and fourth traces have opposite phases for the first two paths whereby the timing at which maximum output occurs is approximately a quarter of the third and fourth components respectively. At the ITU-B wide band chip rate, the spacing between the first two components is approximately three quarters of a chip duration. The power delay profile which has a single peak at the IS 95 chip rate can split into two separate peaks at different timings as the phases are varied in the wide band case.
The present invention seeks to provide timing alignment between orthogonal CDMA channels. The present invention also seeks to provide a technique which allows very rapid and precise control of signal time alignment, with a minimum of signalling overhead. The invention further seeks to provide a CDMA communications arrangement for fixed wireless access systems.
In accordance with a first aspect of the present invention, there is provided a method of determining timing offsets in a CDMA communications link between a base station and a subscriber station, the method comprising the steps of: transmitting a signal by the subscriber using a default timing offset; receiving such signal by the base station and detecting the various multipath components using a Rake receiver, and; when the base station determines that a subscriber station needs to change its timing alignment, it transmits to the subscriber a message identifying a particular timing offset selected from a set of predetermined offsets which the subscriber should use.
The set of predetermined offsets can be stored in a look-up table. The look-up table can be in the form of a volatile storage medium. The look-up table can be in the form of a non-volatile storage medium. The default timing offsets can be those that have been used on a most recent transmission; those that have been used most frequently by the system or predetermined offsets.
Adjustment of the timing can be carried out by the insertion of a short signal into each frame to determine which offset should be selected. Time alignment commands transmitted by the base station may be transmitted once for each transmission or can be transmitted by the base station when signal quality has been degraded. Fine tuning can be performed with small adjustments of time offset, once a particular offset has been selected. The CDMA communications link may be a fixed wireless scheme, wherein the subscriber station is not mobile.
In accordance with a second aspect of the present invention, there is provided a method of operating a CDMA communications link between a base station and a subscriber, the method comprising the steps of: receiving at the subscriber station signals from a base station, referring to time offset data; transmitting signals to the base station using a time offset determined by the time offset data, and; when the base station determines that the time offsets should be adjusted, a pre-determined time offset is selected from the time offset data and such selected pre-determined offset is utilised.
The time offset data can be pre-configured. The time offset data may be determined by experience over a number of iterations and can be updated as appropriate. Preferably, upon initial transmission of a message, the last offset employed by the subscriber station is employed. The CDMA link may be a fixed wireless link.
The speed in creating a communications link can be improved by the time alignment of signals to a predetermined time offset rather than continuously varying the time offset. The timing may be determined as being sub-optimum, but savings in overheads and the convenience in the use with orthogonal sequences can generally be sufficient to negate any such drawbacks. Further, this approach has the benefit of being easily implementable and the degradation from optimum is unlikely to be great.
In accordance with a third aspect of the present invention, there is provided a subscriber station for use in a CDMA communications system between a basestation and a plurality of subscribers, the subscriber station having a look-up table providing a list of time offsets, from which a selected time offset is employed in communication with a base station.
Preferably there is provided means responsive to signals from the base station to change the timing offset to a different timing offset. The time offset can be determined by experience over a number of iterations and can updated as appropriate. The CDMA link may be a fixed wireless link.
In accordance with a fourth aspect of the present invention, there is provided base station for use in a CDMA communications system between a basestation and a plurality of subscribers, the base station having a look-up table providing a list of time offsets, for each active subscriber unit, this table being communicated to a subscriber station upon call initiation or when it is determined the table needs to be updated.
The time offset can be determined by experience over a number of iterations and is updated as appropriate. Alternatively predetermined offsets are employed. The CDMA link may be a fixed wireless link.
In accordance with a still further aspect of the invention there is provided a CDMA communications system operable with predetermined timing offsets.